No. 112 

1961 

AUTOMATIC DEVICES TO TAKE WATER SAMPLES 

AND TO RAISE TRASH SCREENS AT WEIRS 

Experimentation on small watersheds is assuming increasing importance in watershed-management research. Much 

has been accomplished in developing adequate instrumentation 

for use in these experiments. Yet many problems still await 

solution. One difficulty encountered is that small streams 

are subject to wide variations in flow and that these variations are generally unpredictable; so it is often impossible 

for personnel to be on the spot at the right time to take 

samples or to service installations. 

This paper describes devices we have developed to 

take water-quality samples and to manipulate weir trash 

screens. These devices, installed at weirs on the Fernow 

Experimental Forest in West Virginia and the Hubbard Brook 

Experiment a1 Forest in New Hampshire, operate automatically 

when streamflow reaches predetermined stages. 

The Tripping Device 

Both the sediment sampler and the trash-screen lifter 

are activated by a tripping device that operates when the 

water level reaches a predetermined height. The tripping 

device (figs. 1 and 2) consists of a 5-quart can, half filled with concrete, which rides up and down along taut vertical wires fastened to concrete blocks set in the bottom of 

the weir, and overhead to a board that overhangs the top of 

the side wall. These guide wires pass through tubes soldered to opposite sides of the can. 

At low water the can floats about 1 inch above the 

water surface. As the water rises, the can rises with it 

until it strikes an adjustable metal stop. Prevented from 

rising farther, the can then fills with water and sinks to 

the bottom of the weir. 
A cord attached to the can is pulled down by the 

weight of the sinking can. This cord pulls a trigger that 

activates the sediment sampler or the trash-screen lifter. 

Figure 1. --Tripping cans (A) and trash screen (B) in 

place in weir. The trash-screen weight (C) is suspended 

outside the cutoff wall. 

~~:t~~~ tzpping height 
The Sediment Sam~ler 

Sediment discharge from a watershed is generally 

classified as bedload or suspended sediment . Measuring the 

volume of material removed in periodic weir cleanings gives 

a rough estimate of bedload movement of rocks, pebbles, and 

gravel. Soil moved in suspension is measured by periodic 

collection and analysis of water samples. 

Highest concentrations of suspended sediment usually 

occur during rapidly rising stages of streamflow. In remote 

watersheds, these stages may be over before a man can reach 

the sampling point. A single-stage sampler developed at the 

Figure 3. --Sediment sump lers 

in operation. Top: both the 

samplers are empty and ready 

to trip. Bottom: the lowstage sampler has tripped, 

and the hose is resting in 

the flume. 
St. Anthony Falls Hydraulic Laboratory 1 has not been effec- a 

tive in our small turbulent streams; and other automatic 

samplers require electric motors and timer^.^ 

Our sampler operates off a flume of 5-inch gutter 

pipe used to divert a portion of the stream into the sampler, which is located to the rear of the debris basin. The 

sampler consists of a hose, 1 to 13 inches in diameter and 

about 3 feet long, which is attached to a plastic quart bottle, both strapped to a board 1 inch wide (fig. 3). This 

board is balanced on a pivot. 

Figure 4. --Detai 1s of sediment sampler, 

showing pivot point and trigger. 

Prior to sampling, the board, hose, and empty bottle 

are in a nearly vertical position (fig. 3). The sampler is 

held from falling into the flume by a wire joining the bott l e t o a trigger mechanism. This trigger consists of a washer attached to the wire, a nail which passes through the 

washer, and staples that keep the nail in a horizontal 

plane. When the nail is pulled out of the washer, the sampler falls and the hose lands in the water running in the 

'A study of methods used in measurement and analysis of sediment loads in 

streams. Subcommittee on Sedimentation, Federal Interagency Committee on Water 

Resources Report AA. 38 pp., May 1959. 

2 

Developed by the Michigan State Water Resources Commissionj cited by Fa C. 

Tsivoglou, E. D. Harward, and W. M. Ingram in Stream surveys for radioactive 

waste control, Jour. Amer. Water Works Assoc. 49: 750-766, 1957. 
flume. A length of nylon cord (which does not expand in 

water) connects the trigger (fig, 4) to the tripping can, 

When the sampler is tripped (fig. 3), the hose falls 

in the flume, and water runs into the bottle. The device is 

sensitively balanced so that, when the bottle is filled, its 

weight tips the hose upward out of the flume. The open end 

of the hose is cut at a sharp angle to provide an overhang 

that prevents dilution of the sample by rainfall. The 

sample can be poured from the bottle without removing it 

from the board. 

With one sampling rig and one tripping can, it is 

possible to obtain samples at different stages of flow by 

changing the height of the tripping can stop. If samples 

are to be taken at several 'stages during the same storm, a 

battery of samplers can be built. Each requires its own 

tripping can. 

The Trash-Screen Lifter 

Every autumn, in regions having hardwood cover, 

leaves and twigs fall or float into weir basins. In periods 

of low flow, these become lodged in the V-notch and cause 

erroneous readings on the hydrograph. A wire trash screen 

Figure 5. --The trash screen in place. Counterweight and 

trigger for screen lifter are at left. 
placed immediately upstream of the V-notch keeps leaves and e 

twigs out of it. Most screens are rectangular in shape, 

with screen mesh on the top, the two ends, and the upstream 

side. Screens usually are held in place by hooks over the 

weir blade. 

When height of flow increases, the screen must be removed to make certain it does not obstruct flow from the Vnotch and also to prevent formation of an artificial head of 

water behind the debris trapped by the screen. At higher 

flows, leaves and other small debris are usually flushed 

through the notch and cause no malfunction. 

Figure 6. --The trigger. A, its components. B, in position, holding up counterweight. 

In our experience, trash screens should be removed 

when the head of water is between 0.2 and 0.3 foot. Because 

screens must be removed promptly when the water begins to 

rise, it is convenient to have them pulled up automatically. 

Trash screens in use at the Hubbard Brook Experimental Forest are 30 inches long, 12 inches wide, and 9 inches 

high. They are constructed of ;-inch wire mesh. Two floats 

keep the screen riding about half out of the water, and two 

wire hooks over the weir blade keep it centered around the 

notch. A screen of this size does not obstruct discharge up 

to 0.4 foot. 
The automatic screen lifter consists of a counterweight (a cement-f illed bucket, a sash weight, or any\ heavy 

metal weight), attached to the screen by a cord running 

through a pulley mounted about 5 feet above the notch. The 

counterweight, suspended outside the cutoff wall, is kept 

from falling by a trigger. 

The trigger (figs. 5 and 6) consists of two tubes, 

the larger one strapped with a metal band to a board that 

overhangs the cutoff wall and is bolted to the side wall. 

The smaller tube fits loosely into the larger. One end of 

the smaller tube has a notch cut into it, about 1 inch long; 

the other end is attached by a cord to a tripping can. 

A hook (made of a bent f lat-headed nail) is fastened 

with a wire to the counterweight. The flat head of this hook 

is placed in the notch, and the smaller tube is inserted into the larger tube until the hook is engaged in the notch, 

thus suspending the counterweight in the air. When the 

tripping can pulls the small tube back, the hook is released 

from the notch, and the counterweight falls to the ground; 

as it does, the cord, running through the pulley, pulls the 

trash screen up about 2 feet above the side wall of the weir 

and holds it there. 

--K. G. REINHART 

a R. E. L8ONARD 

G. E. HART 

AGRICULTURE-FOREST SERVICE-UPPER DARBY